1. Field of the Invention
The present invention relates to communications systems, particularly to an improved finger assignment method for high data rate (xe2x80x9cHDRxe2x80x9d) calls in a communication system capable of receiving multiple signals.
2. Description of the Related Art
The next generation of wireless networks will provide multiple services requiring high data rate transmission and uninterrupted connections. This next generation is often referred to as the xe2x80x9cthird generationxe2x80x9d of CDMA wireless systems. The range of services include text paging, two-way radio connections, internet connectivity using microbrowsers, two-way wireless e-mail capability and wireless modem functionality. The CDMA cellular telephone system offers the capability to provide reliable radio links between a wireless communications device such as a mobile station (xe2x80x9cMSxe2x80x9d) and a base station (xe2x80x9cBSxe2x80x9d) with a much higher data capacity than conventional networks that only support voice service. As an example, in the third generation CDMA wireless systems, radio links supporting high rate (up to 2 Mbps) data transmissions will be established between the MS and the BS to provide multimedia services such as Internet access.
One particularly important feature of CDMA systems for effective third generation wireless communication is the soft handoff, which allows the MS to move smoothly from the coverage of one cell to another without interruption of service to the user. The soft handoff is accomplished by establishing simultaneous communications between the MS and multiple base stations or BS sectors. In a soft handoff, a MS passes to the edge of the coverage area of a serving BS into a new coverage area of a receiving BS. Momentarily, both BS sectors simultaneously communicate with the MS. As the MS passes further into the coverage area of the receiving BS, the server BS stops communicating with the MS. In this manner, there is uninterrupted communication for the user of the MS as the he or she passes from the server cell to the receiving cell. An efficient soft handoff algorithm plays an important role in maintaining the link quality as well as conserving the capacity-related network resources. As the demand to support high rate data services increases, the need to improve the efficiency of the handoff algorithm becomes more critical.
For a third generation system based on CDMA technologies, a highly efficient handoff algorithm is essential to successfully provide the infrastructure to support the new range of services. A conventional protocol for soft handoffs in a CDMA system has been adopted by the Telecommunications Industry Association in the industry standards IS-95, IS-95 A or IS-95 B (collectively xe2x80x9cIS95 A/Bxe2x80x9d) for implementing a CDMA cellular system. A new feature in the IS-95 B standard not found in IS-95 A is the inclusion of Supplemental Code Channels, or Supplemental Channels within the Traffic Channels. The Traffic Channels are the communication path between the MS and the BS used for user voice and signaling traffic. The term Traffic Channel includes the forward channel from the BS to the MS and the reverse channel from the MS to the BS.
In a code division multiple access (CDMA) cellular telephone system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a mobile station and more than one BS. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from the base stations and the mobile stations. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive a signal from a single transmitting station where the signal has traveled over several distinct propagation paths.
A signal having traveled several distinct propagation paths is generated by the multipath characteristics of the cellular channel. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than other received pulses.
In the mobile radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars and people. In general the mobile radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. Therefore, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time location, attenuation, and phase as a function of the time that the ideal impulse was transmitted.
The multipath characteristic of a channel can result in signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors are added destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of "THgr" radians, and the second path has an attenuation factor of X dB, a time delay of xcex4 with a phase shift of "THgr"+Π radians, no signal would be received at the output of the channel.
In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple paths in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated in the demodulation process. This discrimination not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.
The deleterious effects of fading can be mitigated by controlling transmitter power in the CDMA system. A system for BS and MS power control is disclosed in U.S. Pat. No. 5,056,109 entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,xe2x80x9d issued Oct. 8, 1991, assigned to the Assignee of the present invention. Furthermore the effect of multipath fading can be reduced by communication with multiple base stations using a soft handoff process. A handoff process is disclosed in U.S. Pat. No. 5,101,501 entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d issued Oct. 8, 1991, and assigned to the Assignee of the present invention. The disclosure of U.S. Pat. Nos. 5,056,109 and 5,101,501 are incorporated herein by reference.
A method of assigning multiple demodulation elements or fingers in a spread spectrum system is disclosed in U.S. Pat. No. 5,490,165 (xe2x80x9cthe ""165 patentxe2x80x9d), which disclosure is incorporated as if fully set forth herein. Accordingly, background information and familiarity with the ""165 patent are presumed for the present invention. The ""165 patent is assigned to the Assignee of the present invention.
In the ""165 patent, the MS using a searcher element scans a window of time offsets around the nominal arrive time of each signal of each BS with which active communication is established. The set of base stations having active communication with the MS is called the Active Set. Each scan produces a survey yielding a list of survey paths that comprises pilot signal strength, time offsets, and corresponding BS pilot offset. The survey paths have corresponding data such as the arrival time, signal strength, and transmitter index for each signal. The searcher element passes the information to a controller. The controller tries to match the time offset of each survey path to the time offset of paths currently being demodulated by the fingers. If there are multiple demodulation paths that match one survey path, all fingers or demodulation elements assigned to that path, except the demodulation element having the strongest signal strength indication, are labeled xe2x80x9cfree.xe2x80x9d If a demodulation path exists that does not correspond to a survey path, a survey path entry based on the demodulation path information is added to the list of survey paths.
Next the controller considers the survey paths in order of signal strength with the strongest signal strength survey path being first. If there is no demodulation element assigned to any path in the corresponding sector of the survey path under consideration, the controller attempts to assign a demodulation element to the survey path in the following order. If there is an unassigned or labeled xe2x80x9cfreexe2x80x9d demodulation element, the demodulation element is assigned to the survey path. If no demodulation element is free, the demodulation element having the weakest path that is not the only demodulation path from its BS sector, if any, is re-assigned to the survey path. Finally if the first two cases fail to assign a demodulation element to the survey path, a demodulation element assigned to the weakest path is reassigned to the survey path if the survey path""s signal strength is stronger than the signal strength of the weakest demodulation path. This process continues until one re-assignment occurs or until the last criteria fails to re-assign a demodulation element to the survey path under consideration.
If none of the above rules re-assign a demodulation element for the present survey path, the controller considers the survey paths again in order of signal strength with the strongest signal strength survey path being first. If the survey path is not currently assigned to a demodulation element, the controller may assign any unassigned or labeled xe2x80x9cfreexe2x80x9d demodulation element to the survey path under consideration. If there are no unassigned or labeled xe2x80x9cfreexe2x80x9d demodulation elements, the controller may also re-assign a demodulation element that is assigned to the same BS sector as a survey path if the survey path is stronger than the demodulation path. The controller may also re-assign the weakest demodulation element that is assigned to any BS sector having two or more assigned demodulation elements if the survey path is stronger than the demodulation path. Once either of the two above rules causes a re-assignment or both of the above rules for re-assignment fail for the survey path under consideration, the process begins again with a new scan.
The ""165 patent uses these steps to ensure BS and sector diversity. Each time a demodulation element or finger is re-assigned, a finite time lapses in which no data is demodulated. Therefore, the prior art to the ""165 patent limited the number of demodulation element re-assignments per survey. Comparison ratios are used to create hysteresis in the assignments and thus reduce excessive re-assignment of demodulation elements.
The BS uses a similar but less complicated method to assign the demodulation elements. Because each BS sector receives the same information from a single MS, there is no need to sacrifice the maximum signal level paths to promote diversity. Thus the BS method is based more strictly on signal level while limiting the number of re-assignments per survey similar to the MS method. The BS also uses ratios similar to the mobile station to create hysteresis to reduce excessive re-assignment of demodulation elements
Under the current IS-95 B specifications, a MS may have up to six sectors in its Active Set. The MS may be receiving data at a higher rate on any or all of these sectors. Due to hardware limitations, however, a MS may not have enough demodulating fingers to track all the paths it detects. Therefore, a MS in a soft handoff during a higher data rate call may ignore base stations transmitting on Supplemental Channels under the finger assignment algorithm as disclosed in the ""165 patent.
The Supplemental Channel is an optional portion of a forward or reverse Traffic Channel, which operates in conjunction with a Fundamental Code Channel in the Traffic Channel and optionally with other Supplemental Channels to provide higher data rate services. The Fundamental Code Channel is also a portion of the forward or reverse Traffic Channel which carries a combination of primary data, secondary data, signaling and power control information defined and organized according to the IS-95 B industry standard. The Supplementary Channel transmits a combination of primary data, secondary data, or both, but never signaling formation.
The ""165 patent relates to xe2x80x9cvoice onlyxe2x80x9d systems and therefore does not teach tracking the Supplemental Channels which may be providing supplemental data to the MS separate from a Traffic Channel used for a voice conversation. By not tracking a Supplemental Code Channel, data may be lost when the communication path is temporarily severed during a soft handoff, or when a four way handoff occurs. The following example demonstrates the problem.
FIG. 2 illustrates a four-way soft handoff where the MS has three fingers available to track paths. Base stations A, B, C, and D are all in the Active Sector of the MS on a HDR call. The finger assignment algorithm will select the strongest paths from base stations A, B and C to match up with the fingers since those are the cells indicating the strongest signals. If BS D is the only station transmitting a Supplemental Channel, then the MS will not be demodulating the Supplemental Channel, resulting in an radio link protocol (RLP) resynchronization at the MS.
Data may also be lost or an interruption of communication may occur if the MS is in a handoff and the MS has Y fingers available to track paths, and the BS sectors that are transmitting supplemental channels are the (Y+1)th to last strongest pilots received by the MS. In other words, if the MS has for example 4 fingers available to track paths and the base stations transmitting on supplemental channels is the 5th strongest pilot received by the MS, then the supplemental channel will not be demodulated and the information will not be communicated to the MS under the method taught in the ""165 patent.
In order to address the problem of possibly losing supplemental channels in a soft handoff, the invention disclosed herein is proposed. The present invention insures that at least one finger will be assigned to a cell that is transmitting supplementals, if one exists. If one or more fingers are already demodulating supplemental channels, then the finger assignment algorithm will proceed normally. The present invention provides an improved finger assignment algorithm to that shown in FIGS. 5A-5D of the ""165 patent.
Throughout the disclosure, the use of the terms xe2x80x98demodulation elementxe2x80x99 and xe2x80x98fingerxe2x80x99 are used interchangeably.
According to the present invention, a communication system comprises at least one base station, a mobile station transmitting and receiving communications signals to and from the at least one base station, a control system which assigns fingers to demodulate communication signals received by the mobile station, the control system determining after all the fingers have been assigned to demodulate communication signals whether any of the fingers are demodulating communication signals on supplemental channels. The control system reassigns a finger to demodulate a supplemental channel if no fingers assigned are presently demodulating communication signals on the supplemental channel. One of ordinary skill in the art will understand the necessary implementation of the above structure.
The present invention is also directed to a method of assigning the plurality of fingers to insure that at least one finger will be assigned to a cell or sector that is transmitting on a supplemental channel, if one exists. The method involves the mobile station or receiver having a plurality of fingers or demodulation elements, and assigning the plurality of fingers to a plurality of signals from the base station or base stations. The method comprises measuring the plurality of signals from the base stations beginning with the strongest to the weakest, creating electronic representations of a list of survey paths of each of the plurality of signals, matching a list of demodulation paths corresponding to signals being demodulated by the mobile station to the list of survey paths, determining whether each of the plurality of fingers is matched to one of the plurality of signals, and determining whether any of the fingers is demodulating a supplemental channel. When no fingers are demodulating supplemental channels, the method further comprises determining whether the cell containing the demodulation path being demodulated by the finger supports supplemental channels. If the cell containing the demodulation path being demodulated supports supplemental channels, then the method determines whether the strength of the demodulation path being demodulated is a certain amount greater in strength than the weakest finger, and if yes, the method reassigns that weakest finger to the demodulation path from the cell supporting the supplemental channel. If the strength of the demodulation path with the supplemental channel is not a certain amount greater than the weakest finger, the method begins a new scan of the survey paths beginning with the strongest path.
When there are fingers already demodulating supplemental channels, is the method inquires whether the strength of weakest finger is at least 3 dB weaker than the strength of the demodulation path. If yes, the method determines whether the weakest finger is the only one supporting the supplemental channel. If no, then the method reassigns that finger to the demodulation path. If yes, then the method asks whether the sector containing that demodulation path supports supplemental channels. If yes, then the method reassigns that finger to the demodulation path. If no, the sector does not support supplemental channels, then the method asks whether the call is a high data rate call with supplemental channels and fingers not demodulating supplemental channels. If the strength of the weakest finger is 3dB or less weaker than the demodulation path, the method also determines the same question above, namely, whether the call is a high data rate call with supplemental channels and fingers not demodulating supplemental channels.
In both cases above where the query is whether the call is a high data rate call with supplemental channels and fingers not demodulating supplemental channels, if yes, then the method determines whether there are more paths on the path list. If there are more paths on the path list, the next strongest path on the list is assigned as the demodulation path under consideration and the method continues again as described above. If no, there are no most paths on the path list, then the method continues to assign fingers for path diversity. If the HDR call with supplemental data is being demodulated by a finger, then the method continues to assign fingers for path diversity.
Therefore, if the weakest finger is the only one supporting supplemental channels, and the cell supports supplemental channels, then that finger is assigned to the survey path P with the supplemental channel, although that path P may normally not be assigned a finger because it is the 4th strongest survey path where the mobile station has only 3 fingers or demodulation elements, as an example. In this manner, the present invention insures that the information on a supplemental channel is always demodulated by a demodulation element during a soft-handoff in a high data rate call.